1. Field of the Invention
The present invention relates to a fuel cell stack including a box-shaped casing and a stack body in the casing. The stack body is formed by stacking a plurality of unit cells. Each of the unit cells includes an electrolyte electrode assembly and a pair of separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly which includes an anode and a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. Each of the anode and the cathode is made of electrode catalyst layer of noble metal formed on a base material chiefly containing carbon. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a fuel cell.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Generally, a predetermined number of, e.g., several tens to several hundreds of fuel cells are stacked together to form a fuel cell stack for achieving the desired level of electricity in power generation. Components of the fuel cell stack need to be tightened together so that the internal resistance of the fuel cell does not increase, and the sealing characteristics for preventing leakage of reactant gases can be maintained.
In this regard, a fuel cell stack as disclosed in Japanese laid-open patent publication No. 2001-135344 is known. As shown in FIG. 9, the fuel cell stack includes a stack body 2 formed by stacking a plurality of unit cells 1. End plates 3 are provided at opposite ends of the stack body 2 in the stacking direction. Further, auxiliary plates 4a, 4b are provided outside the end plates 3.
A pair of tightening bands 5 are provided along both sides of the stack body 2. Cylindrical coupling members 6 are provided such that holes of the coupling members 6 are arranged in a line respectively at ends of the tightening bands 5, and the auxiliary plates 4a, 4b. Metal pins 7 are inserted into the holes of the cylindrical members 6. Thus, the tightening bands 5, and the auxiliary plates 4a, 4b are coupled together.
According to the disclosure of Japanese laid-open patent publication No. 2001-135344, a plurality of bolts 8 are screwed into holes of the auxiliary plate 4a, and a plurality of belleville springs 9 are arranged on the auxiliary plate 4b. When the bolts 8 are screwed into the auxiliary plate 4a, the end plate 3 is pressed toward the auxiliary plate 4b, and the belleville springs 9 on the auxiliary plate 4b are compressed. Accordingly, the required tightening force is applied to the stack body 2 between the pair of end plates 3.
However, in the conventional technique disclosed in Japanese laid-open patent publication No. 2001-135344, the end plates 3 may have a rectangular shape. In this case, if a same load is applied to the short sides of the end plates 3 and the long sides of the end plates 3, the deflection in the short sides is different from the deflection in the long sides.
Therefore, if the surface pressure varies in the fuel cell stack due to the thermal expansion, the expansion of the electrolyte membrane, or the creep of the membrane electrode assembly, the uniform surface pressure is not maintained in each of the fuel cells 1. Thus, the power generation performance and sealing characteristics of the fuel cell stack are lowered.